JP4224383B2 - Exhaust purification equipment - Google Patents

Description

  The present invention relates to an exhaust purification device applied to an engine such as a diesel engine.

  Conventionally, a diesel engine is equipped with a selective reduction catalyst having a property of selectively reacting NOx with a reducing agent even in the presence of oxygen in the middle of an exhaust pipe through which exhaust gas flows, and the selective reduction catalyst A required amount of a reducing agent is added to the upstream side of the catalyst so that the reducing agent undergoes a reduction reaction with NOx (nitrogen oxide) in the exhaust gas on the selective catalytic reduction catalyst, thereby reducing the NOx emission concentration. There is what I did.

  For example, as this type of selective reduction catalyst, noble metal catalysts such as platinum and palladium and base metal catalysts such as vanadium, copper and iron oxides are already known as having the above-described properties. The active temperature range (temperature window) of the selective catalytic reduction catalyst is generally narrow, and NOx can be purified only in a part of the exhaust temperature range of the diesel engine. Expansion, especially improvement of low-temperature activity, will become a major challenge in the future.

Therefore, the present inventors have arranged an oxidation catalyst in the preceding stage of the selective catalytic reduction catalyst to oxidize NO in the exhaust gas by the oxidation catalyst to generate strong oxidizing power NO ₂ , By introducing strong NO ₂ to the selective catalytic reduction catalyst, the reduction reaction by the reducing agent on the selective catalytic reduction catalyst is promoted, so that the reduction reaction starts from a lower temperature range than in the case of normal use of the selective catalytic reduction catalyst alone. (See, for example, Patent Document 1).
JP 2002-161732 A

In addition, in the field of industrial flue gas denitration treatment in plants and the like, the effectiveness of a method of reducing and purifying NOx using ammonia (NH ₃ ) as a reducing agent is already widely known. In recent years, it has been difficult to ensure safety when traveling with a toxic substance such as ammonia, and in recent years, the use of non-toxic urea water as a reducing agent has been studied.

However, as a result of diligent research by the present inventors, it is possible to improve the low-temperature activity of the selective catalytic reduction catalyst by equipping the selective catalytic reduction catalyst with an oxidation catalyst, but on the other hand, this type of catalytic oxidation catalyst has a temperature around 300 ° C. Since it has a mountain-shaped catalyst characteristic that peaks at the exhaust temperature, it has been found that NO ₂ is excessively generated at an exhaust temperature around 300 ° C. and the NOx reduction rate drops. It was.

That is, the urea water added to the selective catalytic reduction catalyst has the following formula [Chemical Formula 1] under a temperature condition of about 170 ° C. or higher.
(NH ₂ ) ₂ CO + H ₂ O → 2NH ₃ + CO ₂
As a result of ammonia and carbon dioxide gas, NOx is reduced and purified by this ammonia. However, when NO ₂ is increased by the oxidation catalyst with respect to NO occupying most of the NOx in the exhaust gas, the reaction rate is the highest. The following formula [Formula 2]
NO + NO ₂ + 2NH ₃ → 2N ₂ + 3H ₂ O
As a result, the reduction reaction by NO is promoted, and the reduction of NOx is facilitated.

In order to promote this reduction reaction, it is important that the ratio of NO to NO ₂ in the exhaust gas is close to about 1: 1. However, the oxidation catalyst causes excessive NO ₂ at an exhaust temperature around 300 ° C. Once generated, the ratio of NO ₂ is much higher than the ratio of NO. The excess of NO ₂ is expressed by the following equation [Formula 3].
6NO ₂ + 8NH ₃ → 7N ₂ + 12H ₂ O
As a result, the reaction rate slows down and leaked ammonia that passes through the selective catalytic reduction catalyst unreacted increases. As a result, the NOx reduction rate drops at an exhaust temperature around 300 ° C. It has occurred.

It should be noted that while the ratio of NO ₂ is lower than the ratio of NO, the following equation [Chemical Formula 4]
6NO + 4NH ₃ → 5N ₂ + 6H ₂ O
Alternatively, the following formula [Chemical Formula 5]
4NO + 4NH ₃ + O ₂ → 4N ₂ + 6H ₂ O
As a result, NOx in the exhaust gas is reduced and purified.

The present invention has been made in view of the above-described circumstances, and even if a configuration in which an oxidation catalyst is arranged in the preceding stage of the selective reduction catalyst is used, the generation of excess NO ₂ by the oxidation catalyst is suppressed and NOx is reduced. The purpose is to avoid a drop in the reduction rate.

The present invention is a selective reduction catalyst that is installed in the middle of an exhaust pipe of an engine and can selectively react NOx with ammonia even in the presence of oxygen,
An oxidation catalyst having different NO oxidizing powers installed in parallel in a pair in front of the selective catalytic reduction catalyst;
A branch flow path that distributes exhaust gas to each of the oxidation catalysts and joins them on the inlet side of the selective catalytic reduction catalyst;
Flow path switching means provided at a branch point of the branch flow path;
Urea water addition means for adding urea water as a reducing agent in the exhaust gas on the entry side of the selective catalytic reduction catalyst;
A rotation sensor that detects the number of revolutions of the engine;
A load sensor for detecting the engine load;
If it is determined that there is no risk of excessive NO ₂ generation by the oxidation catalyst based on the detection values from the rotation sensor and the load sensor , the oxidation catalyst having the weaker NO oxidation power is applied. The flow of exhaust gas is blocked, and the entire amount of exhaust gas is led to the selective reduction catalyst through the oxidation catalyst having the stronger NO oxidizing power, while NO ₂ is caused by the oxidation catalyst based on the detection values from the rotation sensor and the load sensor. When it is determined that the operation state is excessively generated, a part of the exhaust gas is distributed to the oxidation catalyst side having a weaker NO oxidizing power, and the amount of NO ₂ produced by both oxidation catalysts is the NO. The flow path switching means is controlled in accordance with the operating state so that the ratio of NO to NO ₂ in the exhaust gas is suppressed to about 1: 1 as compared with the case where only the oxidation catalyst having the stronger oxidizing power is used. Exhaust gas to each oxidation catalyst in the branch channel Exhaust gas purification apparatus characterized by comprising a control device which occupies perform addition of urea water to the urea water addition means while adjusting the metering, in which according to the.

Thus, in this way, when it is determined by the control device that there is no fear of excessive generation of NO ₂ by the oxidation catalyst based on the detection values from the rotation sensor and the load sensor. The control device controls the flow path switching means to block the flow of exhaust gas to the oxidation catalyst having the weaker NO oxidizing power, and the entire amount of exhaust gas is selected through the oxidation catalyst having the stronger NO oxidizing power. Urea water is added to the selective catalytic reduction catalyst by the urea water addition means while being guided to the catalytic reduction catalyst, and NO in the exhaust gas is oxidized when passing through the oxidation catalyst having the stronger NO oxidizing power. is generated as a strong NO ₂ oxidizing power Te, by being guided to such that a strong NO ₂ oxidation power many generated selective reduction catalyst, the reduction with a reducing agent on the selective reduction catalyst Remarkably promoted and normal selection Reduced form made from a low temperature range than in the case of single use of the catalyst as the reduction reaction occurs, NOx in the exhaust gas is to be purified are favorably reduction treatment.

On the other hand, when it is determined by the control device that the NO ₂ is excessively generated by the oxidation catalyst based on the detection values from the rotation sensor and the load sensor , the flow of the exhaust gas is reduced by the flow path switching means. When the NO oxidation power is distributed to the weaker oxidation catalyst side and led to the selective reduction catalyst, the amount of NO ₂ produced by both oxidation catalysts is higher than when only the oxidation catalyst having the stronger NO oxidation power is used. Is suppressed, and the ratio of NO to NO ₂ in the exhaust gas is maintained at about 1: 1, and the NOx reduction rate drops due to excessive generation of NO ₂ at an exhaust temperature around 300 ° C. Will be avoided in advance.

  Furthermore, in the present invention, a temperature sensor for detecting the exhaust temperature is disposed between the oxidation catalyst and the selective catalytic reduction catalyst, or at the outlet of the selective catalytic reduction catalyst, or at both the inlet and outlet of the selective catalytic reduction catalyst, It is preferable to configure the control device so that urea water can be added by the urea water adding means only under the condition that the detection value of the temperature sensor exceeds a predetermined threshold value. It is possible to prevent useless urea water from being added under conditions where the temperature does not reach the activation temperature range of the selective catalytic reduction catalyst.

  According to the exhaust emission control device of the present invention described above, various excellent effects as described below can be obtained.

(I) According to the invention described in claim 1 of the present invention, even if a configuration in which an oxidation catalyst is arranged in the preceding stage of the selective catalytic reduction catalyst, the amount of NO ₂ produced by the oxidation catalyst is appropriately suppressed. As a result, the ratio of NO to NO ₂ in the exhaust gas can be maintained at about 1: 1 which is optimal for the reduction reaction, and the drop in the NOx reduction rate due to excessive generation of NO ₂ can be ensured. It can be avoided.

(II) According to the invention described in claim 2 of the present invention, it is possible to prevent useless urea water from being added under conditions where the temperature of the exhaust gas does not reach the activation temperature range of the selective catalytic reduction catalyst. Further, NOx reduction and purification can be efficiently achieved by adding the minimum amount of urea water.

  Embodiments of the present invention will be described below with reference to the drawings.

1 to 4 show reference examples of embodiments of the present invention. Reference numeral 1 in FIG. 1 denotes an engine that is a diesel engine. In the engine 1 shown here, a turbocharger 2 is provided. The air 4 guided from the air cleaner 3 is sent to the compressor 2a of the turbocharger 2 through the intake pipe 5, and the air 4 pressurized by the compressor 2a is further sent to the intercooler 6 for cooling. The air 4 is guided from the intercooler 6 to an intake manifold (not shown) and introduced into each cylinder of the engine 1.

  Further, the exhaust gas 7 discharged from each cylinder of the engine 1 is sent to the turbine 2b of the turbocharger 2 through the exhaust manifold 8, and the exhaust gas 7 that has driven the turbine 2b goes out of the vehicle through the exhaust pipe 9. It is supposed to be discharged.

  An oxidation catalyst 11 held by a casing 10 is provided in the middle of an exhaust pipe 9 through which the exhaust gas 7 flows. This oxidation catalyst 11 is made of stainless steel by mixing platinum with aluminum oxide (alumina). The structure is supported on a metal carrier or the like.

  The exhaust pipe 9 on the downstream side of the oxidation catalyst 11 is equipped with a selective reduction catalyst 13 held by a casing 12, and this selective reduction catalyst 13 is a flow-through type honeycomb structure. It is formed and has the property that NOx can be selectively reacted with ammonia even in the presence of oxygen.

Here, the selective reduction catalyst 13, platinum, or a noble metal catalyst such as palladium, vanadium, copper, it is possible to adopt a conventionally known catalyst such as a base metal catalyst of oxides of iron, SO ₂ It is more preferable to employ a base metal catalyst having a relatively weak oxidizing power than to employ a noble metal catalyst that easily oxidizes to sulfate (sulfate).

  A position immediately before the casing 10 in the exhaust pipe 9 and a position immediately before the casing 12 behind the casing 10 are connected by a bypass flow path 14, and the exhaust gas 7 from the engine 1 is passed through the bypass flow path 14. The oxidation catalyst 11 can be bypassed and guided to the selective reduction catalyst 13.

  Here, switching valves 15 and 16 (flow path switching means) are respectively provided at the branch passages of the bypass flow path 14 with respect to the exhaust pipe 9 so that the flow of the exhaust gas 7 can be appropriately switched to the bypass flow path 14 side. Yes.

  Further, the engine 1 is equipped with a rotation sensor 17 for detecting the engine rotation speed, and an acceleration sensor 18 for detecting the rotation speed signal 17 a from the rotation sensor 17 and the accelerator opening as a load of the engine 1. A load signal 18 a from the (load sensor) is input to the control device 19.

Then, in the control device 19, as shown in a specific control procedure in FIG. 2, the current rotational speed of the engine 1 is read based on the rotational speed signal 17a from the rotational sensor 17 in step S1, while In step S2, the current fuel injection amount is converted based on the load signal 18a from the accelerator sensor 18. Based on the current engine speed and fuel injection amount, the switching valves 15 and 16 are opened in step S3. The degree of opening at which the ratio of NO to NO ₂ in the exhaust gas 7 is about 1: 1 is read from the two-dimensional map for degree control, and this is opened toward the switching valves 15 and 16 in the next step S4. The degree command signals 15a and 16a are output, and the distribution amount of the exhaust gas 7 to the bypass passage 14 is adjusted.

That is, if the current rotational speed of the engine 1 and the fuel injection amount can be grasped, the flow rate and exhaust temperature of the exhaust gas 7 can be roughly estimated, so that the entire amount of the exhaust gas 7 in the current operating state is passed through the oxidation catalyst 11. In such a case, it is possible to know how the ratio of NO and NO ₂ changes by comparing with preliminary experimental data, etc., and the ratio of NO ₂ is higher than the ratio of NO in any operating condition. Furthermore, when the ratio of NO ₂ exceeds the ratio of NO, if the exhaust gas 7 is diverted to the bypass flow path 14 side by what distribution amount, NO and NO ₂ Whether the ratio can be maintained at 1: 1 can be determined from collation with preliminary experimental data or the like. Therefore, the opening degree control of the switching valves 15 and 16 for realizing this distribution amount is controlled according to the engine speed and the fuel injection amount. Preset as a dimensional map Fluff, it's becomes possible to suppress only production of excessive NO ₂ by the oxidation catalyst 11 reads the control opening based from the two-dimensional map of the rotational speed and the fuel injection amount of the engine 1.

However, in determining the opening degree of the switching valves 15 and 16 so that the ratio of NO to NO ₂ in the exhaust gas 7 is about 1: 1, a λ sensor, a temperature sensor, A NOx sensor may be provided, and the opening degree of the switching valves 15 and 16 may be determined based on these actually measured values.

  It should be noted that there may be a difference (displacement) between the temperature of the exhaust gas 7 and the temperature of the oxidation catalyst 11 at the time of transition in which the operation state is greatly changed. A NOx sensor and a temperature sensor (which may be diverted from the temperature sensor 20 described later) are disposed downstream of the sensor, and the opening degree of the switching valves 15 and 16 determined as described above is corrected as appropriate based on these measured values. After that, the opening command signals 15a and 16a may be output.

  Further, the exhaust pipe 9 between the oxidation catalyst 11 and the selective reduction catalyst 13 is equipped with a temperature sensor 20 for detecting the temperature of the exhaust gas 7 flowing through the exhaust pipe 9. The temperature signal 20a is input to the control device 19.

  Further, the vicinity of the inlet of the selective reduction catalyst 13 in the exhaust pipe 9 and the urea water tank 21 provided at a required place are connected by a urea water supply pipe 22, and the urea water supply pipe 22 is equipped in the middle. By driving the supply pump 23, urea water 24 (reducing agent) in the urea water tank 21 can be added to the vicinity of the inlet of the selective catalytic reduction catalyst 13 via the injection nozzle 25.

  The supply pump 23 for injecting the urea water 24 is driven by a drive command signal 23a from the control device 19, and step S11 is shown as a specific control procedure in FIG. In step S12, the current rotational speed of the engine 1 is read based on the rotational speed signal 17a from the rotational sensor 17, while the current fuel injection amount is converted based on the load signal 18a from the accelerator sensor 18 in step S12. Based on these current engine speed and fuel injection amount, an appropriate drive time is read from the two-dimensional map for drive control of the supply pump 23 in step S13, and this is the next step S15 after step S14. A drive command signal 23a is output to the supply pump 23, and the urea water 24 is added by driving the supply pump 23 for an appropriate time. The amount is adapted to be adjusted.

  That is, if the current rotational speed of the engine 1 and the fuel injection amount can be grasped, the estimated NOx generation amount in the current operating state can be determined by comparison with preliminary experiment data, and the total amount of NOx generated can be reduced and purified. The amount of urea water 24 required for the addition is known from comparison with preliminary experimental data and the like, so that the drive control of the supply pump 23 for realizing this amount of addition is a two-dimensional map of the engine speed and the fuel injection amount. If it is set in advance, it is possible to inject the urea water 24 with an appropriate addition amount simply by reading the control time from the two-dimensional map based on the rotational speed of the engine 1 and the fuel injection amount.

  Here, in step S14 interposed between step S13 and step S15, the current exhaust gas temperature is read based on the temperature signal 20a from the temperature sensor 20, and the detected value of the temperature sensor 20 is a predetermined value. Only when the threshold value (about 170 ° C.) is exceeded, the process proceeds to step S15. When the threshold value is not more than this threshold value, the process proceeds to step S16 and the output of the drive command signal 23a is stopped.

  That is, if the current exhaust temperature does not reach the activation temperature range of the selective catalytic reduction catalyst 13, no reduction reaction occurs on the selective catalytic reduction catalyst 13 even if the urea water 24 is added. The urea water 24 can be added only under the condition that the catalyst reaches the activation temperature range of the selective catalytic reduction catalyst 13 (the condition that the detection value of the temperature sensor 20 exceeds a predetermined threshold). It is.

1 is provided immediately after the selective catalytic reduction catalyst 13 in the casing 12, and a small amount of leaked ammonia that has passed through the selective catalytic reduction catalyst 13 unreacted is converted into NO and N ₂ . An oxidation catalyst for oxidation treatment is shown, and the possibility that ammonia may remain in the exhaust gas 7 finally discharged into the atmosphere by the oxidation catalyst 26 can be avoided.

Thus, if the exhaust gas purification apparatus is configured in this way, NO ₂ may be generated excessively by the oxidation catalyst 11 based on the rotation speed signal 17a of the rotation sensor 17 and the load signal 18a of the accelerator sensor 18. When the control device 19 determines that there is no operation state, the switching valve 15 is closed and the switching valve 16 is opened by the opening command signals 15a and 16a from the control device 19, and the bypass flow path 14 side is closed. The entire amount of the exhaust gas 7 is led to the selective reduction catalyst 13 through the oxidation catalyst 11.

  Further, based on the temperature signal 20 a from the temperature sensor 20, the drive command signal 23 a is sent from the control device 19 to the supply pump 23 only under the condition that the current exhaust temperature reaches the activation temperature range of the selective catalytic reduction catalyst 13. As a result, an appropriate amount of urea water 24 is added to the selective catalytic reduction catalyst 13.

When the urea water 24 is added to the selective reduction catalyst 13 while the exhaust gas 7 from the engine 1 is guided to the selective reduction catalyst 13 through the oxidation catalyst 11 in this way, NO in the exhaust gas 7 passes through the oxidation catalyst 11. oxidized to the time is generated as a strong NO ₂ oxidizing power by being guided to selective reduction catalyst 13 such that a strong NO ₂ oxidation power many are generated, the on the selective reduction catalyst 13 The reduction reaction by the urea water 24 is remarkably accelerated, and the reduction reaction starts from a lower temperature range than in the case of using the normal selective catalytic reduction catalyst 13 alone, and the NOx in the exhaust gas 7 is well reduced and purified. Will be.

On the other hand, when it is determined by the control device 19 that the oxidation catalyst 11 is in an operation state in which NO ₂ is excessively generated, the switching valve 15 is appropriately set by the opening command signals 15a and 16a from the control device 19. The switching valve 16 is throttled by an appropriate opening degree and the flow of the exhaust gas 7 is distributed to the bypass flow path 14 side, and a part of the exhaust gas 7 bypasses the oxidation catalyst 11 to selectively reduce the catalyst. 13 leads to.

In this way, when a part of the exhaust gas 7 from the engine 1 is led to the selective catalytic reduction catalyst 13 by bypassing the oxidation catalyst 11, the amount of NO ₂ generated by the oxidation catalyst 11 is suppressed and the exhaust gas 7 in the exhaust gas 7 is reduced. The ratio of NO to NO ₂ will be maintained at about 1: 1.

  As a result, as shown in the graph of FIG. 4, in the conventional example A in which the oxidation catalyst 11 is provided in the preceding stage of the selective catalytic reduction catalyst 13 and the entire amount of the exhaust gas 7 is allowed to flow through the oxidation catalyst 11, it is about 200 ° C. While a remarkable drop in the NOx reduction rate was confirmed in the temperature range from ˜about 300 ° C., in this embodiment B, the NOx reduction rate dropped in the temperature range from about 200 ° C. to about 300 ° C. Was confirmed to be avoided.

As described above, according to the present reference example , even if the configuration in which the oxidation catalyst 11 is arranged in the preceding stage of the selective reduction catalyst 13 is employed, the oxidation catalyst 11 is in an operation state in which NO ₂ is excessively generated. In this case, a part of the exhaust gas 7 can be led to the selective reduction catalyst 13 by bypassing the oxidation catalyst 11 via the bypass flow path 14, so that the amount of NO ₂ generated by the oxidation catalyst 11 can be appropriately suppressed. Thus, the ratio of NO to NO ₂ in the exhaust gas 7 can be maintained at about 1: 1 which is optimal for the reduction reaction, and the drop in the NOx reduction rate due to excessive generation of NO ₂ can be ensured. It can be avoided.

FIG. 5 shows an example of an embodiment for carrying out the present invention. In the reference example shown in FIGS. 1 to 4, the bypass flow path 14 that bypasses the oxidation catalyst 11 and leads to the selective reduction catalyst 13 is provided. Instead, the casing 10 holds two oxidation catalysts 11A and 11B (an oxidation catalyst 11A having a strong NO oxidation power and an oxidation catalyst 11B having a low NO oxidation power) having different NO oxidation powers in front of the selective catalytic reduction catalyst 13. The exhaust gas 7 is distributed to the respective oxidation catalysts 11A and 11B through the branch flow path 27 so as to be merged on the inlet side of the selective catalytic reduction catalyst 13, and the ratio of the distribution amount Is performed by the switching valves 15 and 16 (flow path switching means) as in the previous embodiment.

Thus, if the exhaust gas purification apparatus is configured in this way, NO ₂ may be generated excessively by the oxidation catalyst 11 based on the rotation speed signal 17a of the rotation sensor 17 and the load signal 18a of the accelerator sensor 18. When the control device 19 determines that there is no operation state, the switching valve 15 is closed and the switching valve 16 is opened by the opening command signals 15a and 16a from the control device 19, and the NO oxidizing power is weaker. The flow of the exhaust gas 7 to the oxidation catalyst 11B is blocked, and the entire amount of the exhaust gas 7 is guided to the selective reduction catalyst 13 through the oxidation catalyst 11A having the stronger NO oxidizing power.

  Further, based on the temperature signal 20 a from the temperature sensor 20, the drive command signal 23 a is sent from the control device 19 to the supply pump 23 only under the condition that the current exhaust temperature reaches the activation temperature range of the selective catalytic reduction catalyst 13. As a result, an appropriate amount of urea water 24 is added to the selective catalytic reduction catalyst 13.

When the urea water 24 is added to the selective reduction catalyst 13 while the exhaust gas 7 from the engine 1 is guided to the selective reduction catalyst 13 through the oxidation catalyst 11A in this way, NO in the exhaust gas 7 passes through the oxidation catalyst 11A. oxidized to the time is generated as a strong NO ₂ oxidizing power by being guided to selective reduction catalyst 13 such that a strong NO ₂ oxidation power many are generated, the on the selective reduction catalyst 13 The reduction reaction by the urea water 24 is remarkably accelerated, and the reduction reaction starts from a lower temperature range than in the case of using the normal selective catalytic reduction catalyst 13 alone, and the NOx in the exhaust gas 7 is well reduced and purified. Will be.

On the other hand, when it is determined by the control device 19 that the oxidation catalyst 11 is in an operation state in which NO ₂ is excessively generated, the switching valve 15 is appropriately set by the opening command signals 15a and 16a from the control device 19. The switching valve 16 is opened only by the opening degree and the switching valve 16 is throttled by an appropriate opening degree, and the flow of the exhaust gas 7 is distributed to the oxidation catalyst 11B side having the weaker NO oxidizing power, and the amount of NO ₂ produced by both oxidation catalysts 11A and 11B Is suppressed more than when only the oxidation catalyst 11A is used, and the ratio of NO to NO ₂ in the exhaust gas 7 is maintained at about 1: 1.

Therefore, according to the above embodiment, when the NO ₂ is excessively generated by the oxidation catalyst 11A having the strong NO oxidizing power, the exhaust gas 7 is partially oxidized with the weak NO oxidizing power. Since it can be distributed to the catalyst 11B and guided to the selective reduction catalyst 13, the amount of NO ₂ produced by the two oxidation catalysts 11A and 11B can be appropriately suppressed, and the ratio of NO to NO ₂ in the exhaust gas 7 can be reduced. It can be maintained at about 1: 1 which is optimal for the reduction reaction, and a drop in the NOx reduction rate due to excessive NO ₂ generation at an exhaust temperature around 300 ° C. can be reliably avoided.

  The exhaust emission control device of the present invention is not limited to the above-described embodiment. The flow path switching means does not necessarily have to be composed of a pair of switching valves, for example, it is composed of a single three-way switching valve. In addition to arranging a temperature sensor between the oxidation catalyst and the selective catalytic reduction catalyst, a temperature sensor is provided at the outlet of the selective catalytic reduction catalyst, or at both the inlet and outlet of the selective catalytic reduction catalyst. Of course, various modifications may be made without departing from the scope of the present invention.

It is the schematic which shows the reference example of the form which implements this invention. It is a flowchart regarding control of a flow-path switching means. It is a flowchart regarding control of urea water addition means. It is the graph which compared this reference example and the prior art example about NOx reduction rate. It is the schematic which shows an example of the form which implements this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Engine 7 Exhaust gas 9 Exhaust pipe 11 Oxidation catalyst 11A Oxidation catalyst with stronger NO oxidation power 11B Oxidation catalyst with less NO oxidation power 13 Selective reduction catalyst 14 Bypass flow path 15 Switching valve (flow path switching means)
16 switching valve (channel switching means)
17 Rotation sensor 18 Acceleration sensor (load sensor)
19 control device 20 temperature sensor 21 urea water tank (urea water addition means)
22 Urea water supply pipe (Urea water addition means)
23 Supply pump (urea water addition means)
24 Urea water (reducing agent)
25 Injection nozzle (urea water addition means)

Claims (2)

A selective reduction catalyst that is installed in the exhaust pipe of the engine and can selectively react with ammonia even in the presence of oxygen;
An oxidation catalyst having different NO oxidizing powers installed in parallel in a pair in front of the selective catalytic reduction catalyst;
A branch flow path that distributes exhaust gas to each of the oxidation catalysts and joins them on the inlet side of the selective catalytic reduction catalyst;
Flow path switching means provided at a branch point of the branch flow path;
Urea water addition means for adding urea water as a reducing agent in the exhaust gas on the entry side of the selective catalytic reduction catalyst;
A rotation sensor that detects the number of revolutions of the engine;
A load sensor for detecting the engine load;
If it is determined that there is no risk of excessive NO ₂ generation by the oxidation catalyst based on the detection values from the rotation sensor and the load sensor , the oxidation catalyst having the weaker NO oxidation power is applied. The flow of exhaust gas is blocked, and the entire amount of exhaust gas is led to the selective reduction catalyst through the oxidation catalyst having the stronger NO oxidizing power, while NO ₂ is caused by the oxidation catalyst based on the detection values from the rotation sensor and the load sensor. When it is determined that the operation state is excessively generated, a part of the exhaust gas is distributed to the oxidation catalyst side having a weaker NO oxidizing power, and the amount of NO ₂ produced by both oxidation catalysts is the NO. The flow path switching means is controlled in accordance with the operating state so that the ratio of NO to NO ₂ in the exhaust gas is suppressed to about 1: 1 as compared with the case where only the oxidation catalyst having the stronger oxidizing power is used. Exhaust gas to each oxidation catalyst in the branch channel Exhaust gas purification apparatus characterized by comprising a control device which occupies perform addition of urea water to the urea water addition means while adjusting the metering. A temperature sensor for detecting the exhaust gas temperature is arranged between the oxidation catalyst and the selective catalytic reduction catalyst, or at the outlet of the selective catalytic reduction catalyst, or at both the inlet and outlet of the selective catalytic reduction catalyst. 2. The exhaust emission control device according to claim 1 , wherein the control device is configured so that the urea water can be added by the urea water adding means only under a condition that exceeds the threshold value.

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